An experimental study has been carried out on coherent forward scattering of microwaves from ion acoustic waves in alkali plasmas in a nonlinear regime. The experimental apparatus allowed quantitative measurements to be made with good accuracy. Geometrical and diffraction effects have been avoided by using a plasma filled waveguide. The dependence of scattering cross section on the amplitude of ion acoustic waves and on the frequency of the incident radiation has been studied in detail. The experimental results are in good agreement with the theoretical results.